CPT Custom category

category.php

<?php if (have_posts()) : while (have_posts()) : the_post(); $post_type = get_post_type( get_the_ID() );

    if ($post_type == 'case_study') ?>
    <div class="col-sm-4 col-xs-12">
      <a href="<?php the_permalink(); ?>" class="gallery-box">
        <figure>
          <?php the_post_thumbnail('medium', array('class' => 'gallery__img')); ?>
          <figcaption class="gallery__caption">
            <div class="gallery__caption-in">
              <h3 class="gallery__tag"><?php the_title(); ?></h3>
              <h4 class="gallery__intro"><?php the_excerpt(); ?></h4>
              <img src="<?php echo get_template_directory_uri(); ?>/img/white-arrow.png" alt="" class="gallery__icon">
            </div>
          </figcaption>
        </figure>
      </a>
    </div>

    <?php endwhile; ?>
    <?php endif; ?>

functions.php

add_filter('pre_get_posts', 'query_post_type');
function query_post_type($query) {
  if( is_category() ) {
    $post_type = get_query_var('post_type');
    if($post_type)
        $post_type = $post_type;
    else
        $post_type = array('nav_menu_item', 'post', 'case_study'); // don't forget nav_menu_item to allow menus to work!
    $query->set('post_type',$post_type);
    return $query;
    }
}